The present invention relates to a method and an apparatus for improving the operation of a disc filter. In particular the invention relates to increasing the discharge consistency of fiber suspensions treated with disc filters in the paper and pulp industry.
Disc filters have been known and used for decades, for example in the wood processing industry. For instance the disc filters are disclosed in British patent specification No. 1,146,197 and in U.S. Pat. No. 3,193,105 are typical examples of this. Even the construction of the filters has during recent years become nearly uniform throughout the industry. In recent times the major improvements in disc filters have been in the development of new materials. Even in the earliest disc filters, a jet of water or corresponding liquid was used to detach the cake of pulp from the surface of a filter sector. For a layman this may seem illogical as the consistency of the pulp cake is of course reduced when liquid is added to it. There have been attempts to detach the cake with air but that has proved to be more expensive than the cost of using water and the cost resulting from the dilution of the pulp caused by the use of water. Thus the users of disc filters have been compelled to accept the fact that even though the consistency of the pulp cake can be raised to 15-16% on the surface of the filter sector, the consistency of the cake after being detached from the filter, e.g. measured at the discharge screw, is only 11-12%.
When studying modern disc filters it has been discovered that the pulp cake is quickly detached by itself by the force of gravity, if the upper corner of the cake has been separated from the surface of the filter sector. However, all of the disc filters currently available are so constructed that the jet of liquid detaching the pulp cakes from the disc continuously sprays liquid onto the filter surface. Most of this liquid of course passes through the filter surface but part of it is immediately and deleteriously absorbed by the pulp cake, the consistency of which is thus reduced. As mentioned before the consistency of the pulp cake is thus reduced by several per cent which is detrimental to further treatment of the pulp. Also, even though the detaching liquid jet passes through the filter surface when the pulp cake is no longer attached at that point to the filter surface, a major part of the liquid runs back through the filter surface as there is no suction inside the filter sector to remove the liquid to the filtrate.
We have discovered that the regulation of the volume of the detaching liquid jet will achieve savings in the cost of pumping of the detaching liquid and will also desirably result in the maintenance of the consistency of the pulp to the maximum consistency reached by the filter itself.
Performed tests have shown that it is possible to use the detaching liquid jet intermittently so as to apply it only for about one third of the time. Hence the amount of the liquid to be injected is reduced to a third and it is estimated that the volume of the liquid absorbed by the pulp cake is reduced to about one-half of the volume absorbed with conventional injection method. Thus if the consistency in the detaching stage is reduced by conventional methods by 4 per cent, the reduction of the consistency with the method and the apparatus of the present invention is only approximately 2 per cent.
The method of improving the operation of a disc filter according to the present invention is characterized in that the detaching pressure medium jet is only intermittently on, thereby resulting in a reduction in the use of detaching water whereby the consistency of the pulp discharged from the disc filter is remarkably increased.
The apparatus for improving the operation of a disc filter according to the present invention is characterized in that means for making the pressure medium jet intermittent is disposed in the pressure medium pipe line supplying the nozzle which sprays the detaching liquid, or is in connection with the nozzle itself.